In various kinds of industries, belt press in which a continuous long object to be pressed is put on a press belt and the object is pressed between one press member positioned inside the press belt and the other press member positioned outside the press belt is used. Here, the press member is a press roll or a press shoe. As an example of the belt press, there is a shoe press as dewatering press in the paper industry.
The shoe press will be briefly described taking the paper industry as an example. It is a pressing (drying) method in which an object (wet paper web) to be pressed is put on an outer peripheral surface of a press belt and pressed between a press roll positioned outside the press belt and serving as external pressing means and a press shoe positioned inside the press belt and serving as internal pressing means, through the press belt. While linear pressure is applied to the object to be pressed in roll press using two rolls, area pressure can be applied to the object to be pressed by using a press shoe having a predetermined width in a travel direction in the shoe press. Therefore, when the dewatering press is performed by the shoe press, a nip width can be increased and a dewatering effect can be enhanced.
In order to make the shoe press compact, as disclosed in Japanese Unexamined Patent Publication No. 61-179359, for example, a shoe press roll in which a press shoe serving as internal pressing means is covered with a cylindrical flexible press belt (press jacket) has been widely used.
Other than the above dewatering process, in the paper industry, the magnetic recording medium manufacturing industry, the fiber industry and the like, the shoe press is performed instead of the roll press or together with the roll press in order to improve quality of an object to be pressed like a calendaring process in which a surface of the object to be pressed is smoothed and glossed. General required characteristics of the press belt includes strength, abrasion resistance, flexibility, impermeability to water, oil, gas and the like. As a material having the above characteristics, polyurethane which is provided by reacting urethane prepolymer with a curing agent is used in the press belt in general. However, since excessive bending or pressing is repeated on the press belt, especially the shoe press belt, a crack which is likely to be generated in the outer peripheral surface is a big problem in view of resistance.
As a method of solving the above problem, Japanese Unexamined Patent Publication No. 10-298893 discloses a shoe press belt in which abrasion resistance and crack resistance is improved by differentiating hardness of a resin which constitutes the belt so as to be high in a center region and low in both edge regions containing parts corresponding to shoe edges in a width direction. In this case, it is thought that there is an effect that the abrasion resistance or pressed deformation resistance is maintained in the center region and the crack is not easily generated in both edge regions.
The crack is likely to be generated intensively in both-end corresponding regions which correspond to both ends of the pressing means in the width direction such as the press roll or the press shoe. Thus, it is thought that the crack resistance is not needed so much in the center region positioned between the both-end corresponding regions and serving as a pressing surface of the object to be pressed, but the abrasion resistance or the pressed deformation resistance should be concerned in the center region.
Although a patent document 2 (Japanese Unexamined Patent Publication No. 10-298893) is provided based on the above thought, in order to satisfy both of the abrasion resistance and the crack resistance by differentiating hardness, it is necessary to largely differentiate the hardness of the center region from that of both edge regions. When the hardness of polyurethane is differentiated, contractive force at the time of processing is varied. Therefore, according to the belt in which the hardness in the center region is largely differentiated from that in both edge regions in the width direction, cylindricality is lowered and traveling performance could be lowered.
In another prior art document, a crack is prevented from being generated in both-end corresponding regions which correspond to both ends of pressing means by improving a configuration or a structure of a press belt or differentiating a depth of a drainage groove (water-squeeze groove). For example, according to Japanese Unexamined Patent Publication No. 2002-180393, a thickness of a middle layer of a press belt positioned so as to correspond to both ends of pressing means in a width direction is increased. In addition, according to Japanese Unexamined Patent Publication No. 2002-327389, a bottom of a drainage groove positioned so as to correspond to both ends of pressing means in a width direction is brought to come close to a reinforcing layer of a middle layer. According to U.S. Pat. No. 5,943,951, flexibility is enhanced by varying a thickness of both ends of a press belt in a width direction gradually. According to U.S. Pat. No. 6,030,503, many recessed parts which are different from grooves are provided in both ends of a press belt in a width direction. According to Japanese Unexamined Patent Publication No. 11-12975, a depth of a drainage groove in both ends of a press belt in a width direction is reduced.